1. Field of the Invention
2. Brief Description of the Prior Art
Diagnosability of diseases is very much dependent on obtaining information about the structures, as well as changes, of tissues of the profound layers that are not primarily accessible. In addition to palpating, exposing or puncturing these tissues, such information can be gained using sophisticated imaging methods such as X-raying, magnetic resonance tomography, or ultrasonic diagnosis.
As biological tissue shows a relatively high permeability for long wave light in the range of 650–1000 nm, a diagnostician can therefore use a completely different method of tissue imaging. The fact that light in the near infrared range can permeate through several centimetres of tissue is utilized in transillumination imaging. This technique as yet facilitates diagnosis of inflammations of the paranasal and maxillary sinuses as well as the detection of accumulated fluids or blood in superficial zones of tissue (Beuthan J., Müller G.; Infrarotdiaphanoskopie, Med. Tech. 1 (1992) 13–17).
Attempts at detecting breast tumours have been unsatisfactory so far (Navarro, G. A.; Profio, A. E.; Contrast in diaphanography of the breast; Med. Phys. 150 (1988) 181–187; Aspegren, K.; Light Scanning Versus Mammography for the Detection of Breast Cancer in Screening and Clinical Practice, Cancer 65 (1990) 1671–77) but there may be better results in the future due to most recent engineering progress (Klingenbeck J.; Laser-Mammography with NIR-Light, Gynäkol.-Geburtsh.-Randsch. 33 Suppl.1 (1993) 299–300); Benaron D. A.; Optical Imaging reborn with technical advances, Diagnostic Imaging (1994) 69–76).
In addition to detecting non-absorbed radiation, fluorescence radiation emitted after near infrared light treatment can provide tissue-specific information. This so-called autofluorescence is used to distinguish atherosclerotic and normal tissue (Henry, P. D. et al., Laser-Induced Autofluorescence of Human Arteries, Circ. Res. 63 (1988) 1053–59).
The main problem of applying near infrared radiation is the extraordinarily wide scattering of the light which permits only a rather blurred image of a clearly contoured object despite different photophysical properties. The problem increases the greater the distance from the surface is and may be considered the major limiting factor of both transillumination and detection of fluorescence radiation.
Suitable fluorescent dyes that accumulate in diseased tissue (above all, in tumours) and that show a specific absorption and emission behaviour, may contribute towards enhancing the distinction of healthy from diseased tissue. The change caused by absorbing irradiated (scattered) light, or fluorescence induced by exciting radiation, is detected and provides the actual tissue-specific information.
Examples of using dyes for in-vivo diagnostics in humans are photometric methods of tracing in the blood to determine distribution areas, blood flow, or metabolic and excretory functions, and to visualize transparent structures of the eye (ophthalmology). Preferred dyes for such applications are indocyanine green and fluorescein (Googe, J. M. et al., Intraoperative Fluorescein Angiography; Ophthalmology, 100, (1993), 1167–70).
Indocyanine green (Cardiogreen) is used for measuring the liver function, cardiac output and stroke volume, as well as the blood flow through organs and peripheral blood flows (I. Med. 24(1993)10–27); in addition they are being tested as contrast media for tumour detection. Indocyanine green binds up to 100% to albumin and is mobilized in the liver. Fluorescent quantum efficiency is low in a hydrous environment. Its LD50 (0.84 mmol/kg) is great enough; strong anaphylactic responses may occur. Indocyanine green is unstable when dissolved and cannot be applied in saline media because precipitation will occur.
Photosensitizers designed for use in photodynamic therapy (PDT) (including haematoporphyrin derivatives, photophrin II, benzoporphyrins, tetraphenyl porphyrins, chlorines, phthalocyanines) were used up to now for localizing and visualizing tumours (Bonnett R.; New photosensitizers for the photodynamic therapy of tumours, SPIE Vol. 2078 (1994)). It is a common disadvantage of the compounds listed that their absorption in the wavelength range of 650–1200 nm is only moderate. The phototoxicity required for PDT is disturbing for purely diagnostic purposes. Other patent specifications dealing with these topics are: U.S. Pat. No. 4,945,239; WO 84/04665, WO 90/10219, DE-OS 4136769, DE-PS 2910760.
U.S. Pat. No. 4,945,239 describes a great number of equipment arrangements for detecting breast cancer using transillumination and mentions the known fluorescein, fluorescamin, and riboflavin as contrast-improving absorption dyes. These dyes share the disadvantage that they absorb in the visible wavelength range of 400–600 nm in which light transmission capacity of tissue is very low.
DE-OS 4136769 describes an apparatus for detecting fluorescence of tissue areas enriched with fluorescent substances. These substances are bacterial chlorophyll and its derivatives, and naphthalocyanines. These structures show absorptions in the range of 700–800 nm at absorbency indices of up to 70000 mol−1 cm−1. In addition to their fluorescent properties, the compounds mentioned here are capable of generating singlet oxygen by radiation, thus having a cytotoxic effect (photosensitizers for photodynamic therapy). This photosensitizing activity is highly undesirable for a pure, inactive diagnostic agent.
Furthermore, synthesis of bacterial chlorophyll compounds is expensive and requires much effort as natural products have to be used as parent substances; the naphthalocyanines, however, frequently show a very low photostability. The known compounds of these classes are hardly soluble in water, and synthesizing uniform hydrophilic derivatives is costly.
WO 84/04665 describes an in-vivo method for the fluorescence detection of tumours using the following photosensitizers: haematoporphyrin and its derivative(Hp and HpD), uro- and copro- and protoporphyrin as well as numerous mesosubstituted porphyrins, and dyes such as riboflavin, fluorescein, acridine orange, berberine sulfate and tetracyclines. The photophysical and pharmacochemical requirements mentioned above are not met by said substances.
Folli et al., Cancer Research 54, 2643–2649 (1994), describe a monoclonal antibody connected with a cyanine dye that was used for detecting a tumour implanted subcutaneously. Detection of profounder pathologic processes, however, requires much improved dyes. Higher dye dosages render the use of antibodies as carriers unsuitable in view of the side effects to be expected.
Cyanine dyes and polymethine dyes related to them are also used as photographic layers. Such dyes need not have any luminescent properties. Cyanine dyes that have luminescent (fluorescent) properties have been synthesized for use in fluorescent microscopy and flow cytometry and coupled with biomolecules such as compounds containing iodine acetyl groups as specific labelling reagents for sulfhydryl groups of proteins (Waggoner, A. S. et al.; Cyanine dye Labeling Reagents for Sulfhydryl Groups, Cytometry, 10, (1989), 3–10). Proteins are labelled and isolated in this way. More references: Cytometry 12 (1990) 723–30; Anal. Lett. 25 (1992) 415–28; Bioconjugate Chem. 4 (1993) 105–11.
DE-OS 39 12 046 by Waggoner, A. S. describes a method for labelling biomolecules using cyanine and related dyes such as merocyanine and styryls that contain at least one sulfonate or sulfonic acid grouping. This specification relates to a single and two-step labelling method in a hydrous environment, with a covalent reaction taking place between the dye and the amine, hydroxy, aldehyde or sulfhydryl group on proteins or other biomolecules.
DE-OS 3828360 relates to a method for labelling antitumour antibodies, in particular, antibodies specific to melanoma and colonic cancer, using fluorescein and indocyanine green for ophthalmologic purposes. Bonding of indocyanine green to biomolecules is not covalent (dye-antibody combination, mixture).
The known, state-of-the-art methods of in-vivo diagnosis using NIR radiation thus show a number of disadvantages that prevented their wide application in medical diagnostics.
Direct use of visible light or NIR radiation is restricted to superficial body zones, which is due to the widely scattered incident light.
Adding dyes to improve contrast and resolution, however, gives rise to a number of other problems. The dyes should meet the requirements that generally apply to diagnostic pharmaceuticals. As these substances are mostly applied at higher doses and for a longer diagnostic period, they should be low-toxic. In addition, dyes suitable for diagnostic purposes should be well soluble in water and sufficiently stable in chemical and photophysical respect, at least for as long as the diagnostic period lasts. Stability as regards metabolization in the system is also desirable.
So far, neither dyes nor a suitable method for in-vivo diagnosis using NIR radiation have been available.
It is therefore an object of this invention to provide a method of in-vivo diagnosis that overcomes the disadvantages of prior art. dr